Weather experts say it’s time to get ready for a winter whiplash as the polar vortex is about to elbow wet and warm El Nino out

WASHINGTON Get ready for weather whiplash as powerful climatic forces elbow each other for starring roles in a weird winter show.

The spine-chilling polar vortex is taking centre stage in Europe and bringing persistent cold to much of North America ‚Äî except in Hollywood, where soggy El Nino won’t give up the spotlight.

El Nino delivered a balmy Christmas Eve to the eastern U.S. and shattered national records with a warm, wet and wild December, but meteorologists say the situation is about to change.

“The biggest thing is this whiplash,” said University of Oklahoma meteorology professor Jason Furtado. “It’s going to be a shock for people.”

The centre of the cold blast starts this weekend in the U.S. in the upper Midwest, and then moves to the Hudson Bay area next week, while in Europe it starts in the east and north and then spreads, Furtado said. Europe may have to get used to temperatures 20 or so degrees below normal.

“Temperature will be dominated by the impact of the polar vortex,” said Judah Cohen, seasonal forecast chief for the private Atmospheric and Environmental Research company outside Boston. It will feel similar to 2013 and 2014, he said.

Americans became painfully familiar with the polar vortex during those winters. This time, America’s winter temperatures will depend on when the wet and warm El Nino pushes itself back to prominence, Furtado said.

“We have all of these large and unusual events happening all at the same time and I don’t think it has ever happened before,” said Rutgers University climate scientist Jennifer Francis.

For a winter this dramatic, it may help to consult the program:THE CAST

The star is El Nino, a veteran of this stage for a few decades now. This natural warming of the central tropical Pacific occurs every two to seven years or so, and changes weather worldwide, especially in the Americas and Asia. It is closely associated with heavy rain in California, and general warming. It has less effect in Europe because that’s further away. With its flipside, La Nina, it is known as the El Nino Southern Oscillation, and it lasts about a year.

Playing off against El Nino is the Arctic Oscillation, and its index measures differences in atmospheric pressure between the Arctic and mid-latitudes. When the AO is positive in the winter, polar air stays trapped up north and the weather is relatively mild further south. When the AO is negative, the cold Arctic air escapes and plunges into lower latitudes, treating the United States and Europe to the polar vortex ‚Äî a swirling air mass that carries spine-chilling temperatures.

The North Atlantic Oscillation is a bit player with a big influence over Europe’s weather. As with its Arctic cousin, the more negative its index, the colder the continent becomes.

The jet streams also are important to watch. These rivers of air heavily influence local weather everywhere, carrying storms and clearing skies around the planet. Usually in the winter, the jet stream that affects most people in the United States and Europe moves relatively straight from west to east. But when it weakens, it can plunge south and north and even get stuck at times, creating odd extremes.

Another air pattern playing a small but key role is the Madden-Julian Oscillation, which travels in the warm parts of the Indian and Pacific oceans, and may have triggered changes that “flipped the switch” in December, Furtado said.

Still other characters may be a factor in making the jet streams oscillate more wildly: A huge blob of warm water in the northern Pacific, unusually low sea ice levels in the Arctic’s Barents Kara sea area; a cool patch of water off Greenland, likely from melting ice sheets and glaciers; winter storm Frank that hit England; and of course man-made climate change.

ACT ONE

This was last month. El Nino appeared in classic form, tying 1997-1998 for the strongest on record. The Arctic Oscillation was very positive, trapping cold air in northern latitudes. With so many warm, wet days further south, spring flowers popped up and trees bloomed in December.

“It’s not surprising we were 70 degrees on Christmas Day,” said Mike Halpert of the National Oceanic and Atmospheric Administration’s Climate Prediction Center in College Park, Maryland.

El Nino often spawns winter tornadoes in the southern U.S. and they appeared on cue, killing two dozen people in just four days. The heavy rain that flooded the Mississippi River Valley isn’t usually an El Nino signature though, Cohen said.

ACT TWO

The switch flipped: The Arctic Oscillation and its North Atlantic sidekick went negative big time.

This time, there’s cold air, but it’s mostly dry so far, despite El Nino, which still spawns a series of rainstorms hitting California. Meteorologists say wetter weather could bring heavy snowstorms, but that’s not likely for another week or more, although cities along the Great Lakes may get lake effect snow.

So far the east-moving moisture from El Nino is staying south of the Arctic plunge, but that may change.

For now, the AO is dominating in its fight with El Nino, especially in Europe, Cohen said. NFL players and fans will likely brave temperatures around one below zero in Minneapolis.

ACT THREE

No spoilers here. Will the polar vortex hold the stage for weeks or months? Will the cold AO and wet El Nino combine for whopper snowstorms?

Rutgers’ Francis doesn’t think the moisture and the cold can keep avoiding each other, saying “we’re going to have major major dumps of snow. We just don’t know where.”

Furtado predicts El Nino will push aside the cold in America, but not in Europe, where it has less of an influence. Cohen is less certain about what will happen as these forces morph and interact over time.

“There’s a lot going on; the weather has been crazy,” Cohen said. “Expect the unexpected.”